Current measuring apparatus and method

ABSTRACT

The disclosure describes an improved apparatus for measuring the flow of current through a conductor by means of a Hall generator. The Hall generator is biased by a constant current source employing feedback. In order to increase the accuracy of the current measurement, operational amplifiers having inputs which float in relationship to the power supply operating the amplifiers are used to amplify the Hall generator output voltage and to drive a meter. The current measuring apparatus can be used to detect defects in a vehicular alternator by measuring the fluctuating components of the alternator output current. The magnetic field generated by the current to be measured is concentrated in the area of the Hall generator by means of a unique hollow, cylindrical, coiled strip of metal made from nickel, molybdenum and iron. The apparatus includes a regulator in which the voltage drop is so low that a 3.25 volt regulated supply can be maintained from a poorly-charged 6-volt battery.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates to current measuring techniques, and moreparticularly relates to current measuring techniques employing a Hallgenerator.

Hall generators for measuring the current carried through a conductorhave been known in the past. However, the accuracy of such devices hassuffered in the past due to inadequate regulating, current biasing andreadout circuits associated with the Hall generator. Although coiledmetal strips having the configuration described herein have been used inthe past in connection with Hall generators, the fabrication of thecoils from silicon steel has limited their usefulness.

Accordingly, it is an object of the present invention to provide animproved constant current circuit for biasing a Hall generator by meansof a sample voltage proportional to the current flowing through the Hallgenerator, a reference voltage, and means for comparing the referenceand sample voltages.

Another object of the invention is to provide a bias circuit of theforegoing type in which the Hall generator and an adjustable portion ofthe circuit are held within a probe which can be placed around theconductor carrying the current to be measured, and in which theremaining portion of the bias circuit is contained in a separate cabinetconnected to the probe through a cable, so that Hall generators withdifferent characteristics can be adjusted to work interchangeably withlike bias circuits.

Still another object of the present invention is to provide a readoutfor the Hall generator in which the Hall output voltage is conducted tothe inputs of at least one operational amplifier which floats inrelationship to the power supply operating the amplifier.

Yet another object of the present invention is to provide a readoutcircuit of the foregoing type in which a meter for indicating thecurrent to be measured is connected in the feedback loop of anoperational amplifier having inputs adapted to receive the Hall voltage.

Another object of the present invention is to provide a method ofdetecting defects in a vehicular alternator by measuring the fluctuatingcurrent produced by the alternator.

Still another object of the present invention is to provide a method ofthe foregoing type in which the fluctuating current is attenuated belowa predetermined frequency.

Yet another object of the present invention is to provide a probe of theforegoing type in which the magnetic field produced by the current to bemeasured is concentrated in the area adjacent the Hall generator bymeans of a toroidal, laminated core comprising nickel and iron.

Another object of the invention is to provide an improved regulator inwhich the voltage drop is sufficiently low to enable operation from apartially discharged vehicular battery.

DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention willhereafter appear in connection with the accompanying drawings in whichlike numbers refer to like parts throughout, and in which:

FIG. 1 is a schematic drawing illustrating a preferred form of cabinet,meter and probe made in accordance with the present invention;

FIG. 2 is a schematic drawing of a typical vehicular AC charging systememploying an alternator which generates current that can be measured bythe apparatus of the present invention;

FIG. 3 is a cross-sectional, fragmentary view of a portion of the probeshown in FIG. 1;

FIG. 3A is an enlarged, fragmentary elevational view of the circledportion of FIG. 3;

FIG. 3B is a side elevation of the strip metal coil forming a part ofthe probe shown in FIG. 1;

FIGS. 4A and 4B are electrical schematic drawings of a preferred form ofcircuitry made in accordance with the present invention;

FIG. 5 is an equivalent circuit showing the components which areconnected when the switch shown in FIG. 4B is moved to the DC currentmeasuring positions 284 or 285; and

FIG. 6 is an equivalent circuit illustrating the components connectedtogether when the switch shown in FIG. 4B is moved to the fluctuatingcurrent measuring position 286.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 2 illustrates a typical vehicular charging system 10 whichgenerates current that can be measured by the preferred embodiment ofthe invention. System 10 includes a rechargeable lead-storage battery 12having a positive terminal 13, a negative terminal 14 and a ground cable16 that is connected to the chassis of the vehicle. Battery 12 is aconventional lead-acid battery which is typically used to provide astarting current for automobiles.

Battery 12 is charged by current carried through a charging conductor 17which is attached to a terminal 19 of a starter switch 20. The currentalso is carried through a conductor 21 connected to the output terminal22 of an alternator 23. The alternator is a conventional AC generatorwhich includes rectifying diodes in order to produce a three-phaserectified DC current. The amount of current produced at terminal 22 isdetermined by the amount of current flowing into a field windingterminal 24 from a voltage regulator 26. A voltage is received byvoltage regulator 26 from the battery through an ignition switch (notshown) and a conductor 27. A conductor 28 provides a DC current to thevarious portions of the vehicle, such as the lights, ignition system andaccessories (not shown).

As shown in FIG. 1, a preferred form of the present invention basicallycomprises a probe assembly 40 and a circuit system 100 held in a cabinet102. The cabinet and probe are connected by a cable 85.

More specifically, referring to FIGS. 1 and 3, probe assembly 40comprises a clamp 42 including an upper member 44 having a semi-circularjaw 45 with an inside diameter 46 and an outside diameter 47. The uppermember also includes a handle 49. Clamp 42 further comprises a lowermember 54 having a semi-circular jaw 55 with an inside diameter 56 andan outside diameter 57. A handle 59 cooperates with handle 49 to allowthe jaws to be opened and closed by rotating them around a pivot axis61. The jaws normally are biased in the closed position shown in FIG. 1by means of a spring 62. Upper and lower jaws 45 and 55 hold a toroidal,laminated core fabricated in the form of a hollow, cylindrical, coiledstrip of metal 66 having an inner convolution 67i and an outerconvolution 67o. Referring to FIG. 3B, the strip of metal is woundaround a circular mandrel (not shown) having a diameter as large asconvolution 67i so that the strip takes the form of a ring. Electricallyinsulating adhesive is applied to the strip as it is wound on themandrel. After baking, the adhesive holds the parts together andinsulates the convolutions which form the lamina of the coil. After thestrip is coiled and baked, it is cut in half along a diameter 68, and aradial slot 69 is formed. The cutting operation divides coil 66 into anupper semicircular segment 66a and two lower segments 66b and 66c. Aftercoil 66 is cut along diameter 68, upper segment 66a is placed in jaw 45and lower segments 66b, 66c of the coil are placed in jaw 55. As shownin FIG. 3A, plastic spacers 70, 71 maintain the proper spacing of slot69.

Spring 62 normally biases the upper and lower segments of the coiltogether, so that the segments touch each other on both sides, as shownin FIG. 1. When handles 49 and 59 are moved together, the upper andlower segments of coil 66 move apart so that a current-carryingconductor, such as conductor 17, can be conveniently moved inside thecoil.

Coil 66 is made from Mollypermalloy which contains approximately 79percent nickel, 17 percent molybdenum, and the balance iron. The coil ishydrogen annealed to provide a very round hysteresis loop. The purposeof the annealing is to provide minimum hysteresis. Hysteresis wouldcause the coiled strip to retain some of the magnetic field to which ithad been previously exposed. Thus, any exposure to a strong field wouldleave a residual magnetism which would give a reading that would have tobe canceled out prior to the next measurement. Due to the features ofcoil 66, probe 40 has inherently low hysteresis. The coiling of the corelowers the hysteresis, and the composition of the material and annealingalso aid in the process.

As shown in FIGS. 3A and 3B, a Hall generator 72 is mounted in slot 69so that it is completely sealed in the body of the probe. Hall generator72 is a four-terminal device having four output conductors or leads74-77 which are wired to a printed circuit board 79. Hall generator 72is a square of silicon doped to a predetermined conductivity anddeposited on a ceramic wafer. The silicon is quite thin, and contactsare made at four points on the periphery. These four points, in turn,are connected to leads 74-77, respectively.

To operate the Hall generator, a bias current is conducted through leads74 and 75. The current of a conductor placed inside coil 66 (e.g.,conductor 17) produces a magnetic field having a vector normal to theplane of Hall generator 72. A corresponding output voltage proportionalto the magnetic field appears across Hall generator outputs 76, 77. Theoutput voltage is proportional to the product of the bias current andthe normal components of the magnetic field. If the magnetic field isnot normal to the Hall generator, but instead is at some angle theta,then the normal component will be the actual field vector times thecosine of theta. The Hall generator responds only to the normalcomponent. The constant of proportionality takes into account thegeometry, the thickness, and the doping of the Hall generator itself,and has a temperature coefficient which is approximately -0.07% perdegree Centigrade. Since the Hall generator has a negative temperaturecoefficient, compensation is provided to make the coefficient as near tozero as possible. It has been discovered that this result can beachieved if a temperature dependent resistor is placed near the Hallgenerator in the probe. This arrangement is particularly important forvehicle current measuring applications in which the probe temperaturemay vary drastically from the cabinet temperature. For example, cabinet102 may be in a hot room and probe 40 may be under the hood of an icecold vehicle, or vice versa.

Referring to FIGS. 3 and 4B, circuit board 79 holds a 5K potentiometer80 and a temperature-dependent resistor, such as a thermistor 82. Anadditional resistor 83 is also mounted on board 79. Probe 40 and cabinet102 are connected together by a cable 85 comprising conductors 87-91(FIG. 4B). As described in detail later, the location of potentiometer80 in probe 40 is an important feature which enables each Hall generatorto be individually calibrated so that any probe can be interchangeablyused with any circuit system of the type shown in the drawings.

Referring to FIG. 1, cabinet 102 includes a meter 104 having a face 106calibrated in amperes and a pointer 107. The meter includes a coil ofwire (not shown) having a resistance which varies with temperature.Meter 104 is a zero center variety which reads left of center forcurrents in one direction and right of center for currents in theopposite direction.

The circuitry in system 100 for indicating current flow on meter 104based on the output voltage produced by Hall generator 72 basicallyincludes a regulator system 120, a current bias system 190, and areadout system 230 Hall generator 72, together with regulator system 120and current bias system 190, form a Hall system for producing a Hallvoltage proportional to the magnitude of a current flowing throughconductor 17.

Referring to FIG. 4A, regulator system 120 receives input voltage frombattery 12 over input conductors 122 and 123. Conductor 122 is connectedto the positive terminal of battery 12, whereas conductor 123 isconnected to the negative terminal of battery 12. Conductor 123, thus,becomes a negative power bus or terminal. System 120 utilizes thevoltage received from battery 12 in order to produce a regulated voltageon power supply bus or terminal 125.

System 120 includes a series regulating transistor 127 having a baseelement 129, an emitter element 130, and a collector element 131connected as shown. Transistor 127 is fitted with an appropriate heatsink in order to dissipate power as heat energy.

Regulator system 120 includes a filter and protection circuit comprisinga diode 134 and capacitors 136-138. Diode 134 protects the circuitry incase leads 122, 123 are connected backwards. Capacitor 136 acts as aspike filter to remove spikes that may be present from the battery ofthe vehicle under test. Capacitors 137, 138 act as filters to insure alow output impedance of the regulator at higher frequencies.

The regulator also includes a sensing circuit 140 comprising resistors142, 143, and a potentiometer 144 having a slider 145. Circuit 140samples the voltage maintained on supply bus 125.

Sensing circuit 140 acts in cooperation with a reference circuit 148including a resistor 149 and an integrated circuit reference diode 150which produces a 1.22 volt drop. Diode 150 may be implemented by aNational Semiconductor Corp. integrated circuit model LM113H.

The sensing and reference circuits provide inputs to a comparatorcircuit 152 comprising an operational amplifier 154 connected as adifference amplifier. The amplifier includes an inverting input 155, anon-inverting input 156, a feedback capacitor 157 and an output resistor158 through which a correction signal is transmitted. The correctionsignal is processed by an amplifier 160 including a transistor 162 and aresistor 164 connected as shown.

Due to the unique configuration of regulating transistor 127, theregulator includes a starting circuit 166 comprising switch transistors168, 169 and diodes 170, 171. The diodes establish a switching voltagewhich switches transistor 168 into a conductive state when the regulatoris initially connected to a battery. Transistor 169 is operated bysensing resistors 173, 174. The junction between the resistorsestablishes a sensing voltage proportional to the supply voltage on bus125. Additional resistors 176-178 are connected as shown.

An important feature of regulator system 120 is its ability to maintaina 3.25 volt regulated supply on bus 125 from a partially discharged6-volt lead storage battery. Under conditions of heavy load or partialdischarge, the battery voltage may be as low as 4.5 volts. It has beendiscovered that the power drain from the battery and the voltage droprequired by regulating transistor 127 both can be reduced to a minimumby using the unique configuration of components shown. According to onefeature of the circuit, the emitter of transistor 127 is operativelyconnected to the positive terminal of the battery, whereas the collectorof transistor 127 is operatively connected to the regulated supply bus125. This arrangement minimizes the voltage drop required by transistor127 in order to achieve adequate regulation.

When battery 12 is initially connected to regulator system 120, startingcircuit 166 causes transistor 127 to begin to conduct current from itsemitter 130 to its collector 131. As the voltage on conductor 122 rises,current flows through resistor 177, and a voltage is developed at thebase of transistor 168 due to the voltage drop across diode 170, 171. Asa result, transistor 168 is switched to its conductive state so that thecurrent is drawn through resistor 176. The flow of current creates avoltage drop across resistor 176, and transistor 127 begins to conductcurrent from its emitter to collector elements. The base of transistor168 is clamped to approximately 1.2 volts by the drop across diodes 170,171. As the collector of transistor 127 rises in voltage, voltage isapplied to resistors 173, 174. When the junction of resistors 173, 174exceeds the 1.2 volt drop across diodes 170, 171, transistor 168 isswitched to its non-conductive state, and transistor 169 is switched toits conductive state. In this mode of operation, transistor 169 becomesa load for transistor 127, and the variable collector-emitter impedanceof transistor 127 then is adjusted by comparator circuit 152. Inputs155, 156 of operational amplifier 154 compare the voltages produced bysensing circuit 140 and reference circuit 148. Amplifier 154 produces acorrection signal which is amplified by transistor 162, and the outputof transistor 162 is applied to the base of transistor 127. Thecorrection signal varies the emitter-collector impedance of transistor126 to maintain 3.25 volts on supply bus 125.

Another novel feature of this regulator is its ability to operate safelyfrom power supplies with continuous levels as high as +40 volts. If +40volts is applied to input conductor 122, up to 40 volts will be appliedto capacitor 136; resistors 164, 176 and 177; and transistors 127, 162and 168. These components have been selected to withstand voltages up to+40 volts continuous, and all other components in the circuit areprotected since the collector of transistor 127 will remain at +3.25volts. The current drawn through supply bus 125 will not appreciablyincrease because transistor 127 is a series regulator, and, afterstartup is achieved, transistor 168 is switched to its non-conductivestate. Transistor 162 conducts only enough current to operate transistor127 irrespective of the voltage appearing on conductor 122. Thus, theonly appreciable current increase is through resistor 177, and theincrease is limited to a few milliamperes.

Current bias system 190 includes a reference voltage circuitincorporating potentiometer 80 which is located in probe 40, as well asa 1K resistor 192 located in cabinet 102. The potentiometer acts as asource of adjustable voltage in probe 40 which can initially adjust thebias current flowing through Hall generator inputs 74, 75.

The reference circuit also includes resistors 194, 195 arranged as avoltage divider. Voltage at the junction of resistors 194, 195 combineswith the adjustable voltage from potentiometer 80 and resistor 192 togenerate a reference voltage. A sensing resistor 197 is connected inseries with the Hall generator input terminals 74, 75. Resistor 197generates a sensing voltage proportional to the current flowing throughterminals 74, 75.

A current control circuit 200 utilizes the sensing voltage and referencevoltage in order to maintain the current flowing through the Hallgenerator at a constant value which can be initially adjusted bypotentiometer 80. Circuit 200 includes an operational amplifier 202connected as a difference amplifier having an inverting input 203, anon-inverting input 204 and an output 205. A transistor 207 is connectedas an amplifier in order to control the current flowing through the Hallgenerator 72. Transistor 207 can be considered as part of the differenceamplifier incorporating operational amplifier 202. The current controlalso includes diodes 209, 210, resistors 212, 213 and capacitors 215-217connected as shown. Resistor 212 insures that transistor 207 will notturn on through its leakage current or through the leakage current ofoperational amplifier 202.

When the output of operational amplifier 202 decreases toward zerovolts, electron current flows through diodes 209, 210 into resistor 213and the base of transistor 207. As a result, the electron currentflowing from emitter to collector of transistor 207 increases in orderto increase the bias current flowing through Hall generator 72. If theoutput of operational amplifier 202 increases toward the 3.25 voltsupply, the operation is reversed so that the electron current flowingfrom emitter to collector of transistor 207 is decreased. This mode ofoperation can be used in order to maintain the bias current flowingthrough the Hall generator at a preadjusted level.

The manner in which this operation is achieved can best be explained byassuming that potentiometer 80 is adjusted to its full resistance of5000 ohms. Then, the reference voltage at the inverting input 203 ofoperational amplifier 202 is approximately 0.25 volts. The bias currentthrough terminals 74, 75 of Hall generator 72 increases until thevoltage across resistor 197 is 0.25 volts, at which time the current isapproximately 16.7 milliamperes, the minimum bias current. By theoperation previously described, operational amplifier 202 and transistor207 maintain the bias current at 16.7 milliamperes irrespective oftemperature and supply voltage changes.

If the resistance value of potentiometer 80 is decreased, more currentis fed back through resistor 192, and the reference voltage at invertinginput 203 is increased. As a result, more current flows through leads74, 75 of Hall generator 72 until the voltage across resistor 197 equalsthe increased reference voltage. If potentiometer 80 is adjusted to zeroohms, the current through leads 74, 75 of Hall generator 72 isapproximately 42 milliamperes, the maximum bias current.

The ability of potentiometer 80, located in probe 40, to adjust the biascurrent created and regulated by circuit 190 in cabinet 102 is animportant feature. It allows each probe to be initially adjusted so thatits bias current times its Hall generator coefficient yields a constantoutput voltage across leads 76, 77. This adjustment is achieved byplacing the jaws of the probe around a conductor carrying a known amountof current. Potentiometer 80 is then adjusted until pointer 107indicates the correct number of amperes on meter face 106. In otherwords, potentiometer 80 is adjusted so that the output voltage from eachHall generator will be identical for a given number of amperes flowingthrough a conductor located inside coil 66 of probe 40. The probesadjusted in this manner become interchangeable, and can be attached toany cabinet 102 containing the type of current bias system 190 shown inFIG. 4A. If such an arrangement were not available, the difference insensitivity in the Hall generators would make it impossible tointerchange the probes without completely recalibrating each currentbias system. It would be impossible to send a new probe to a user in thefield. Instead, an entire cabinet and matched, calibrated probe wouldneed to be dispatched.

Referring to FIG. 4B, readout system 230 conditions the voltage producedacross leads 76, 77 of Hall generator 72 in order to drive meter 104.The system includes an operational amplifier 232 having an invertinginput 233, a non-inverting input 234 and an output 235. The operationalamplifier receives voltage from bus 125 through a positive supplyterminal 236 and receives ground potential through a negative supplyterminal 237 connected to conductor 123.

Operational amplifier 232 is provided with three different feedbacknetworks depending on the type of readout desired. A first feedbacknetwork includes a resistor 238 and a capacitor 239; a second feedbacknetwork includes resistors 241, 242 and a capacitor 243; and a thirdfeedback network includes resistors 245, 246, 247 and a capacitor 248arranged as a filter. Operational amplifier 232 also is associated withresistors 250-255, potentiometers 257-259 and capacitors 261, 262connected as shown.

Readout system 230 also includes an operational amplifier 266 having aninverting input 267, a non-inverting input 268 and an output 269.Positive voltage is supplied to operational amplifier 266 from supplybus 125 through a positive terminal 270, and ground potential issupplied to the amplifier from conductor 123 through a negative supplyterminal 271.

Operational amplifier 266 is connected as a meter drive circuit. Inorder to achieve this result, meter 104 is connected in a feedback loopof amplifier 266, together with a full wave rectifier 274 comprisingdiodes 275-278. A resistor 280 is connected from output 269 to groundpotential. Resistors 255 and 280 are used to increase the currentsinking capability of their respective amplifiers 232 and 266.

The operating functions of readout system 230 are controlled by a switchassembly 282 comprising contacts 284a-284d used in conjunction with the500 ampere scale on meter face 106, contacts 285a-285d used inconjuction with the 100 ampere scale on meter face 106 and contacts286a-286d used in conjunction with the diode stator scale on meter face106. The contacts are selected by ganged slides 290a-290d which areoperated in synchronism by test selector knob 292 (FIG. 1).

Assuming the current to be measured is from about 100-500 amperes, knob292 and slides 290a-290d are moved to the positions shown in FIGS. 1 and4B. In these positions, the contacts of switch assembly 282 interconnectthe circuit components in the manner shown in FIG. 5. As previouslydiscussed, thermistor 82 is located inside probe 40 in order tocompensate for the decrease in voltage from Hall generator 72 as thetemperature rises. As shown in FIG. 5, lead 76 of Hall generator 72becomes the reference point for the readout system which floats withrespect to the electrical power furnished by bus 125 and groundconductor 123. This is an important feature which gives the readoutsystem common mode rejection of like changes in voltage on leads 76, 77of Hall generator 72. In other words, the reading of meter 104 does notchange substantially if leads 76 and 77 increase or decreasesimultaneously in voltage to the same extent.

As shown in FIG. 5, operational amplifier 232 amplifies the voltageproduced across leads 76, 77 of Hall generator 72. The gain of theamplifier is approximately equal to the value of feedback resistor 238divided by the equivalent resistance of thermistor 82 and resistors 83,252. Typically a gain of 15 to 16 is provided. Capacitor 239 (FIG. 4B)reduces the frequency response, and thereby reduces the noise level. Theoutput of amplifier 232 drives operational amplifier 266 which isconnected as a meter driver. The overall gain of the readout system canbe adjusted by means of potentiometer 259.

The placement of meter 104 in the feedback circuit of driver amplifier266 is an important feature which enables the meter to be accuratelydriven irrespective of changes in meter coil resistance due totemperature variations. When the meter 104 is placed in the feedbackcircuit, the current flowing through the meter equals the input currentflowing into inverting input 267. This current is the Hall outputvoltage across terminals 76, 77 times the gain of the amplifier 232stage divided by the sum of the values of potentiometer 259 and resistor254. At full scale, the current flowing through meter 104 is about 0.5milliamperes.

Another important feature of the readout system is that amplifiers 232and 266 "float" with respect to the DC power supplied by bus 125 andconductor 123. As shown in FIG. 4B, the supply voltage is effectivelyisolated from the inputs and outputs of amplifiers 232 and 266 by thearrangement of the components. This feature substantially improves thecommon mode rejection of the readout system. In other words, when thevoltages on terminals 76 and 77 change simultaneously by substantiallythe same values, the reading on meter 104 remains substantiallyunchanged. This feature, of course, increases the accuracy andreliability of the current measurement.

The circuitry of FIG. 5 uses the voltage produced by the Hall generatorin order to indicate on meter 104 the magnitude of the direct currentflowing through conductor 17.

When knob 292 is moved to the 100 position (FIG. 1) so that slides290a-290d touch contacts 285a-285d, the DC current flowing throughconductor 17 is measured in the same manner described above, except thatthe gain of amplifier 232 is changed by altering the resistance value ofthe feedback loop in order to measure currents in the 0-100 ampererange.

Before beginning any test, meter 104 should be zeroed by operatingpotentiometers 257 and 258 (FIG. 4B). Potentiometer 258 is a sensitivityadjustment device for potentiometer 257 which is connected across leads74, 75 of Hall generator 72. Potentiometer 257 places a slightlyunbalanced resistor network across one side of Hall generator 72 whichcompensates for the zero offset of the generator. The zero offset isinherent in the device due to the impossibility of manufacturing aperfectly symmetrical generator. Potentiometer 257 is adjusted by a zeroknob 294 (FIG. 1) to correct for any small imbalance in the Hallgenerator, thus causing the meter current to be zero when there are zeroamperes flowing in conductor 17.

By moving knob 292 to the ALT position (FIG. 1) so that slides 290a-290dtouch contacts 286a-286d, respectively, the components shown in FIG. 6are connected together. These components are used in order to detectdefective diodes or windings in alternator 23 (FIG. 2) by measuring thefluctuating current flowing through conductor 17 or 21. The test can beconducted without disconnecting or alterating in any way the circuitconnections of the vehicle incorporating alternator 23. Thus, the testcan be conducted under actual operating conditions. This is an importantadvantage over prior alternator testers which have required theinsertion of test components into the vehicle charging system. One suchprior system is shown in U.S. Pat. No. 3,594,642 (Wright-July 20, 1971).

A properly operating alternator will produce a smooth, three-phaserectified current flow, with very little ripple component. An alternatorwith a shorted or open diode or an open stator winding or partiallyshorted stator winding will produce a much higher ripple content. Thecircuitry shown in FIG. 6 detects the increased ripple or fluctuatingcomponent of current produced by alternator 23, and displays it on meter104. If pointer 107 indicates a ripple or fluctuating current componentgreater than R (FIG. 1) on meter 104, a defective alternator isindicated.

As shown in FIG. 6, the feedback network for amplifier 232 is changed toa T-network which maintains a reasonbly low value of DC gain, but veryhigh AC gain. The AC gain is frequency dependent and is determined bythe values of resistors 245-247 and capacitor 248. Another such circuitis formed by resistors 251, 252 and capacitors 261, 262. Due to thesecircuits, the gain of amplifier 232 rolls off or is attenuated at a highrate below approximately 200 Hertz. Likewise, the gain of amplifier 232begins to roll off or attenuate at frequencies greater thanapproximately 2500 Hertz due to the roll off characteristic of theamplifier itself. Due to the DC blocking effect of capacitor 262, thecomponents shown in FIG. 6 form a frequency-shaping circuit which passesonly fluctuating signals, and which provides peak gain at approximately1 K Hertz. The frequency characteristics of the FIG. 6 circuitry rejectsthe thin, evenly-spaced ripple spikes present in a properly functioningthree-phase alternator, and rejects the low frequency hunting caused bythe action of regulator 26 in its effort to stabilize the batteryvoltage. However, the circuit amplifies the fluctuating components ofcurrent due to defects in the alternator. As a result, a substantialmeter current greater than value R flows in meter 104 in reponse to anysuch defect.

In order to measure the DC current flowing through conductor 17 of thevehicle charging system shown in FIG. 2, probe 40 is zeroed in themanner described above. The jaws of probe 40 are opened and placedaround conductor 17. The jaws then are closed so that conductor 17 ispositioned in the manner shown in FIG. 1. Conductor 122 is connected tobattery terminal 13 and conductor 123 is connected to battery terminal14. No part of the vehicle charging system needs to be disconnected.

Knob 292 then is moved to the "500" or "100" position (FIG. 1), and theDC current flowing through conductor 17 is accurately shown on meterface 104. The system continues to operate accurately as long as battery12 can maintain about 4.5 volts across terminals 13 and 14. Of course,the system can be used while the engine of the vehicle is stopped,cranking or running.

In order to test alternator 23, the alternator is driven by the engineand knob 292 is moved to the ALT position (FIG. 1). The connections tothe vehicle described above remain the same. If pointer 107 moves to theright of the R mark, the alternator is defective.

Those skilled in the art will recognize that only a single embodiment ofthe present invention has been illustrated, and that the embodiment maybe modified and altered without departing from the true spirit and scopeof the invention as defined by the accompanying claims.

We claim:
 1. Apparatus for determining the magnitude of a current to bemeasured flowing through a conductor, said apparatus comprising:a probeadapted to be coupled to the conductor; a cabinet; a Hall generatorlocated in the probe, said Hall generator including first and secondHall inputs for receiving a bias current and first and second Halloutputs for producing a Hall voltage proportional to the current to bemeasured; current control means located in the cabinet for controllingthe bias current; utilization means located in the cabinet for utilizingthe Hall voltage; adjustment means located in the probe for enabling thecurrent control means to generate a predetermined value of bias current,so that the Hall voltage is calibrated; and cable means for electricallyinterconnecting the adjustment means to the current control means andfor conducting the bias current and Hall voltage between the cabinet andprobe, whereby each probe can be individually adjusted by the adjustmentmeans to be interchangeably connected to a variety of cabinets throughthe cable means.
 2. Apparatus, as claimed in claim 1, wherein thecurrent control means comprises:reference means for generating areference voltage; sample means for generating a sample voltageproportional to the magnitude of the current flowing through the Hallgenerator; and means for adjusting the bias current flowing from thefirst to second Hall inputs until the reference voltage and samplevoltage attain a predetermined relationship, whereby the bias current ismaintained constant; and wherein the utilization means comprises readoutmeans for indicating the magnitude of the measured current bydetermining the magnitude of the Hall voltage.
 3. Apparatus, as claimedin claim 2, wherein the adjustment means comprises a source ofadjustable voltage located in the probe; and wherein the reference meanscomprises means for combining the adjustable voltage with apredetermined voltage level to form the reference voltage.
 4. Apparatus,as claimed in claim 3, wherein the source of adjustable voltagecomprises a potentiometer connected to the first Hall input. 5.Apparatus, as claimed in claim 4, wherein the means for combiningcomprises a voltage divider including at least first and second dividerresistors connected by a junction attached to the potentiometer. 6.Apparatus, as claimed in claim 5, wherein the sensing means comprises aseries resistor connected in series with the first and second Hallinputs.
 7. Apparatus, as claimed in claim 6, wherein the current controlmeans comprises:a difference amplifier including a first amp input, asecond amp input and an amp output; means for connecting the junction ofthe first and second divider resistors to the first amp input; means forconnecting the series resistor to the second amp input; and means forconnecting the amp output to one of the first and second Hall inputs. 8.Apparatus, as claimed in claim 2, wherein the readout meanscomprises:power supply means for generating a DC voltage between a firstpower terminal and a second power terminal; first operational amplifiermeans including a first inverting input, a first non-inverting input, afirst output, a first positive supply terminal, a first negative supplyterminal, and a feedback loop; first connection means for operativelyconnecting the first power terminal to the first positive supplyterminal; second connection means for operatively connecting the secondpower terminal to the first negative supply terminal; means for couplingan input voltage proportional to the Hall voltage across the firstinverting input and the first non-inverting input; a meter responsive tocurrent including a coil having an impedance; third connection means forconnecting the meter in the feedback loop; and means for isolating thepower supply means from the first inverting input, first non-invertinginput and first output, whereby the reading of the meter remainssubstantially constant irrespective of like changes in voltage on thefirst and second Hall outputs and irrespective of changes in theimpedance of the meter.
 9. Apparatus, as claimed in claim 8, wherein themeans for coupling comprises a temperature dependent resistor located inthe probe.
 10. Apparatus, as claimed in claim 8, wherein the means forcoupling comprises:second operational amplifier means including a secondinverting input, a second non-inverting input, a second output, a secondpositive supply terminal, and a second negative supply terminal; fourthconnection means for operatively connecting the first power terminal tothe second positive supply terminal; fifth connection means foroperatively connecting the second power terminal to the second negativesupply terminal; sixth connection means for operatively connecting thefirst Hall output to the second inverting input and for operativelyconnecting the second Hall output to the first and second non-invertinginputs; seventh connection means for operatively connecting the secondoutput to the first inverting input; and second means for isolating thepower supply means from the second inverting input, second non-invertinginput and second output.
 11. Apparatus, as claimed in claim 8, whereinthe feedback loop is connected between the first inverting input and thefirst output.
 12. Apparatus, as claimed in claim 2, wherein the readoutmeans comprises:amplifier means for amplifying the fluctuatingcomponents of the Hall voltage; rectifier means for converting theamplified Hall voltage to a corresponding DC signal; and meter means forindicating the magnitude of the DC signal, whereby defects in avehicular alternator can be detected.
 13. Apparatus, as claimed in claim12, and further comprising filter means for attenuating the fluctuatingcomponents of the Hall voltage below a first predetermined frequency.14. Apparatus, as claimed in claim 13, wherein the first predeterminedfrequency is about 200 Hertz.
 15. Apparatus, as claimed in claim 1,wherein the probe further comprises a toroidal, laminated core having aninner diameter sufficiently large to receive the conductor and having aradial slot in which the Hall generator is located, said core comprisingnickel and iron.
 16. Apparatus, as claimed in claim 15, wherein the coreis cut in half to allow the insertion of the conductor into the insideof the toroidal core.
 17. Apparatus, as claimed in claim 16, wherein thecore is annealed to reduce hysteresis.
 18. Apparatus, as claimed inclaim 17, wherein the core comprises at least 35 percent nickel. 19.Apparatus, as claimed in claim 17, wherein the core comprises nickel,molybdenum and iron.
 20. Apparatus, as claimed in claim 19, wherein thecore comprises over 60 percent nickel, over 10 percent molybdenum andthe balance iron.
 21. Apparatus, as claimed in claim 18, wherein thecore comprises a hollow, cylindrical, coiled strip of metal having aninsulating adhesive positioned between the convolutions of the coil. 22.In a system for determining the magnitude of a current to be measuredflowing through a conductor by use of a Hall generator including firstand second Hall inputs for receiving a bias current and including firstand second Hall outputs for producing a Hall voltage proportional to thecurrent to be measured, improved apparatus for reading out the Hallvoltage comprising:power supply means for generating a DC voltagebetween a first power terminal and a second power terminal; currentcontrol means for enabling a controlled amount of bias current to beconducted from the first power terminal through the first and secondHall inputs to the second power terminal; first operational amplifiermeans including a first inverting input, a first non-inverting input, afirst output, a first positive supply terminal, a first negative supplyterminal, and a feedback loop; first connection means for operativelyconnecting the first power terminal to the first positive supplyterminal; second connection means for operatively connecting the secondpower terminal to the first negative supply terminal; means for couplingan input voltage proportional to the Hall voltage across the firstinverting input and the first non-inverting input; a meter responsive tocurrent including a coil having an impedance; third connection means forconnecting the meter in the feedback loop; and means for resistivelycoupling the power supply means to the first inverting input, firstnon-inverting input and first output, whereby the reading of the meterremains substantially constant irrespective of like changes in voltageon the first and second Hall outputs and irrespective of changes in theimpedance of the meter.
 23. Apparatus, as claimed in claim 22, whereinthe means for coupling comprises a temperature dependent resistor. 24.Apparatus, as claimed in claim 22, wherein the means for couplingcomprises:second operational amplifier means including a secondinverting input, a second non-inverting input, a second output, a secondpositive supply terminal, and a second negative supply terminal; fourthconnection means for operatively connecting the first power terminal tothe second positive supply terminal; fifth connection means foroperatively connecting the second power terminal to the second negativesupply terminal; sixth connection means for operatively connecting thefirst Hall output to the second inverting input and for operativelyconnecting the second Hall output to the first and second non-invertinginputs; seventh connection means for operatively connecting the secondoutput to the first inverting input; and second means for isolating thepower supply means from the second inverting input, second non-invertinginput and second output.
 25. Apparatus, as claimed in claim 22, whereinthe feedback loop is connected between the first inverting input and thefirst output.